1. Field of the Invention
The invention relates to a process for preparing highly pure polycrystalline silicon in a fluidized-bed reactor by thermal decomposition or reduction, with hydrogen, of a silicon source gas.
2. The Prior Art
Highly pure polycrystalline silicon (polysilicon) is a starting material for the fabrication of electronic components and solar cells. It is obtained by thermal decomposition or reduction, with hydrogen, of a silicon source gas. This process is known to those skilled in the art as chemical vapor deposition, CVD. Worldwide, polysilicon is produced mainly in so-called Siemens reactors. The chemical vapor deposition of elemental silicon in these CVD reactors takes place on silicon rods, so-called thin rods. These rods are heated to more than 1000.degree. C. under a metal bell jar by means of electric current and are then exposed to a gas mixture consisting of hydrogen and a silicon source gas, for example trichlorosilane. As soon as the thin rods have grown to a certain diameter, the process sequence has to be interrupted, i.e. only batchwise operation rather than continuous operation is possible.
Most recently, attempts were made to obtain highly pure polycrystalline silicon as granules, hereinafter referred to as "silicon granules", in fluidized-bed reactors in the course of a continuous CVD process. Fluidized-bed reactors are preferred where solid surfaces are to be exposed extensively to a gaseous or vaporous compound. This will ensure an efficient operation. A gas mixture of a silicon source gas, for example trichlorosilane, and a silicon-free carrier gas, for example, hydrogen are known as the "reaction gas". If this reaction gas perfuses a fluidized bed comprising polysilicon particles, hereinafter referred to as "silicon particles", elemental silicon is deposited on the surface of these particles. These particles as a result, grow in size to produce granular polysilicon, hereinafter referred to as "silicon granules".
To provide the thermal conditions required for the CVD process, the fluidized-bed reactor is usually divided, via partitioning means, into two zones (See DE-43 27 308 C2):
1) a reaction zone within which the silicon source gas is deposited as polysilicon on the silicon particles at a specific reaction temperature, and
2) a heating zone within which a fraction of the silicon particles is fluidized by means of an inert, silicon-free carrier gas and is heated beyond the reaction temperature, for example by means of microwave energy.
In addition to the efficient operation of fluidized-bed reactors, a further advantage is that the polysilicon product is obtained in the form of granules which are approximately spherical. This free-flowing, granular polysilicon can immediately be transported and processed. On the other hand the rod-type product from the Siemens reactor first has to be crushed before it can be processed further, such as with the Czochralski process.
Notwithstanding these advantages, the use of a fluidized-bed reactor for preparing silicon granules often has drawbacks. In contrast to the polysilicon obtained by the Siemens process, the granules can be contaminated with chlorine, if chlorosilanes such as trichlorosilane are used as the silicon source gas. The contamination is trapped in the crystal lattice, and consequently cannot be removed even by annealing under a vacuum.
If there is further processing of the silicon granules by the Czochralski process, then a high chlorine contamination has adverse effects. For example there will be poor crystal quality of the single crystal pulled from the melt, due to the formation of gas bubbles in the silicon melt. There will also be concomitant splashing of the melt, and the formulation of chlorine-containing corrosive gases which destroy the pulling tool. These drawbacks are not observed with a chlorine contamination below 50 ppm by weight.
In principle there is no difference between the deposition of a gaseous silicon source gas on silicon rods in a Siemens reactor and on silicon particles in a fluidized-bed reactor. If the composition and the ratio of the reaction gas, the temperature of the silicon surfaces (rods or particles) and the reactor pressure are the same in both processes, there is the following result. The silicon granules from the fluidized-bed reactor nevertheless exhibit distinctly elevated chlorine contamination, compared with the rod-type polysilicon from the Siemens reactor.
U.S. Pat. No. 5,077,028 discloses that the chlorine contamination correlates inversely with the average deposition rate of elemental silicon on the silicon particles. Thus, as the deposition rate increases, the chlorine contamination decreases. According to this patent, a deposition rate greater than 0.4 .mu.m/min at a temperature greater than 1000.degree. C. is a prerequisite for a granular polysilicon product with a chlorine contamination below 20 ppm by weight.
However, operating a fluidized-bed reactor at a temperature of 1000.degree. C. does result in substantial, undesirable deposition of elemental silicon on the insides of the reactor walls. This causes the termination of the CVD process. Such a high reactor temperature is virtually unachievable industrially and ensures neither reliable nor efficient operation.